Parameterization for fading compensation

ABSTRACT

Techniques and tools for performing fading compensation in video processing applications are described. For example, during encoding, a video encoder performs fading compensation using fading parameters comprising a scaling parameter and a shifting parameter on one or more reference images. During decoding, a video decoder performs corresponding fading compensation on the one or more reference images.

RELATED APPLICATION INFORMATION

This application is a continuation of U.S. patent application Ser. No. 17/012,900, filed Sep. 4, 2020, entitled “PARAMETERIZATION FOR FADING COMPENSATION,” which is a continuation of U.S. patent application Ser. No. 16/290,418, filed Mar. 1, 2019, entitled “PARAMETERIZATION FOR FADING COMPENSATION,” now U.S. Pat. No. 10,805,616, which is a continuation of U.S. patent application Ser. No. 15/809,449, filed Nov. 10, 2017, entitled “PARAMETERIZATION FOR FADING COMPENSATION,” now U.S. Pat. No. 10,264,284, which is a continuation of U.S. patent application Ser. No. 15/149,796, filed May 9, 2016, entitled “PARAMETERIZATION FOR FADING COMPENSATION,” now U.S. Pat. No. 9,843,822, which is a continuation of U.S. patent application Ser. No. 13/584,671, filed Aug. 13, 2012, entitled “PARAMETERIZATION FOR FADING COMPENSATION,” now U.S. Pat. No. 9,363,536, which is a continuation of U.S. patent application Ser. No. 11/838,758, filed Aug. 14, 2007, entitled “PARAMETERIZATION FOR FADING COMPENSATION,” now U.S. Pat. No. 8,265,148, which is a continuation of U.S. patent application Ser. No. 10/378,958, filed Mar. 3, 2003, entitled “PARAMETERIZATION FOR FADING COMPENSATION,” now U.S. Pat. No. 7,277,486, which claims the benefit of U.S. Provisional Patent Application Ser. No. 60/377,628, filed May 3, 2002, the disclosure of which is incorporated herein by reference. This application relates to U.S. patent application Ser. No. 10/378,988, entitled “Fading Estimation/Compensation,” filed Mar. 3, 2003, and U.S. patent application Ser. No. 10/379,079, entitled “Signaling for Fading Compensation,” filed Mar. 3, 2003, the disclosures of which are incorporated herein by reference.

TECHNICAL FIELD

Techniques and tools for parameterization for fading compensation in video processing applications are described. For example, a video encoder performs fading compensation on a reference image based on fading parameters.

BACKGROUND

Digital video consumes large amounts of storage and transmission capacity. A typical raw digital video sequence includes 15 or 30 frames per second. Each frame can include tens or hundreds of thousands of pixels (also called pels). Each pixel represents a tiny element of the picture. In raw form, a computer commonly represents a pixel with 24 bits. Thus, the number of bits per second, or bit rate, of a typical raw digital video sequence can be 5 million bits/second or more.

Most computers and computer networks lack the resources to process raw digital video. For this reason, engineers use compression (also called coding or encoding) to reduce the bit rate of digital video. Compression can be lossless, in which quality of the video does not suffer but decreases in bit rate are limited by the complexity of the video. Or, compression can be lossy, in which quality of the video suffers but decreases in bit rate are more dramatic. Decompression reverses compression.

In general, video compression techniques include intraframe compression and interframe compression. Intraframe compression techniques compress individual frames, typically called I-frames or key frames. Interframe compression techniques compress frames with reference to preceding and/or following frames, which are typically called predicted frames, P-frames, or B-frames.

Microsoft Corporation's Windows Media Video, Version 8 [“WMV8”] includes a video encoder and a video decoder. The WMV8 encoder uses intraframe and interframe compression, and the WMV8 decoder uses intraframe and interframe decompression.

A. Intraframe Compression in WMV8

FIG. 1 shows an example of block-based intraframe compression (100) of a block (105) of pixels in a key frame in the WMV8 encoder. For example, the WMV8 encoder splits a key video frame into 8×8 blocks of pixels and applies an 8×8 Discrete Cosine Transform [“DCT”] (110) to individual blocks, converting the 8×8 block of pixels (105) into an 8×8 block of DCT coefficients (115). The encoder quantizes (120) the DCT coefficients, resulting in an 8×8 block of quantized DCT coefficients (125) which the encoder then prepares for entropy encoding.

The encoder encodes the DC coefficient (126) as a differential from the DC coefficient (136) of a previously encoded neighbor (e.g., neighbor block (135)) of the block being encoded. The encoder entropy encodes the differential (140). FIG. 1 shows the left column (127) of AC coefficients encoded as a differential (147) from the left column (137) of the neighboring (to the left) block (135). The remaining AC coefficients are from the block (125) of quantized DCT coefficients.

The encoder scans (150) the 8×8 block (145) of predicted, quantized AC DCT coefficients into a one-dimensional array (155) and then entropy encodes the scanned AC coefficients using a variation of run length coding (160). The encoder selects an entropy code from one or more run/level/last tables (165) and outputs the entropy code (170).

B. Interframe Compression in WMV8

Interframe compression in the WMV8 encoder uses block-based motion compensated prediction coding followed by transform coding of the residual error. FIGS. 2 and 3 illustrate the block-based interframe compression for a predicted frame in the WMV8 encoder. In particular, FIG. 2 illustrates motion estimation for a predicted frame (210) and FIG. 3 illustrates compression of a prediction residual for a motion-estimated block of a predicted frame.

For example, the WMV8 encoder splits a predicted frame into 8×8 blocks of pixels. Groups of four 8×8 blocks form macroblocks. For each macroblock, a motion estimation process is performed. The motion estimation approximates the motion of the macroblock of pixels relative to a reference frame, for example, a previously coded, preceding frame. In FIG. 2, the WMV8 encoder computes a motion vector for a macroblock (215) in the predicted frame (210). To compute the motion vector, the encoder searches in a search area (235) of a reference frame (230). Within the search area (235), the encoder compares the macroblock (215) from the predicted frame (210) to various candidate macroblocks in order to find a candidate macroblock that is a good match. After the encoder finds a good matching macroblock, the encoder outputs information specifying the motion vector (entropy coded) for the matching macroblock so the decoder can find the matching macroblock during decoding. When decoding the predicted frame (210) with motion compensation, a decoder uses the motion vector to compute a prediction macroblock for the macroblock (215) using information from the reference frame (230). The prediction for the macroblock (215) is rarely perfect, so the encoder usually encodes 8×8 blocks of pixel differences (also called the error or residual blocks) between the prediction macroblock and the macroblock (215) itself.

FIG. 3 illustrates an example of computation and encoding of an error block (335) in the WMV8 encoder. The error block (335) is the difference between the predicted block (315) and the original current block (325). The encoder applies a DCT (340) to the error block (335), resulting in an 8×8 block (345) of coefficients. The encoder then quantizes (350) the DCT coefficients, resulting in an 8×8 block of quantized DCT coefficients (355). The quantization step size is adjustable. Quantization results in loss of precision, but not complete loss of the information for the coefficients.

The encoder then prepares the 8×8 block (355) of quantized DCT coefficients for entropy encoding. The encoder scans (360) the 8×8 block (355) into a one dimensional array (365) with 64 elements, such that coefficients are generally ordered from lowest frequency to highest frequency, which typically creates long runs of zero values.

The encoder entropy encodes the scanned coefficients using a variation of run length coding (370). The encoder selects an entropy code from one or more run/level/last tables (375) and outputs the entropy code.

FIG. 4 shows an example of a corresponding decoding process (400) for an inter-coded block. Due to the quantization of the DCT coefficients, the reconstructed block (475) is not identical to the corresponding original block. The compression is lossy.

In summary of FIG. 4, a decoder decodes (410, 420) entropy-coded information representing a prediction residual using variable length decoding (410) with one or more run/level/last tables (415) and run length decoding (420). The decoder inverse scans (430) a one-dimensional array (425) storing the entropy-decoded information into a two-dimensional block (435). The decoder inverse quantizes and inverse discrete cosine transforms (together, 440) the data, resulting in a reconstructed error block (445). In a separate motion compensation path, the decoder computes a predicted block (465) using motion vector information (455) for displacement from a reference frame. The decoder combines (470) the predicted block (465) with the reconstructed error block (445) to form the reconstructed block (475).

The amount of change between the original and reconstructed frame is termed the distortion and the number of bits required to code the frame is termed the rate for the frame. The amount of distortion is roughly inversely proportional to the rate. In other words, coding a frame with fewer bits (greater compression) will result in greater distortion, and vice versa.

C. Limitations of Conventional Motion-Based Video Compression

Video sequences with effects such as fading, morphing, and blending require relatively large amounts of bits to encode because conventional motion-based video compression methods are generally not effective on such frames. For example, consider a video sequence in which an object in a frame has moved slightly in one direction from one frame to the next. In a typical block-matching motion estimation technique, it may be a simple matter in a video sequence without fading to find a good match in the previous frame for a block in the current frame and encode the resulting motion vector. However, if, for example, a “fade-to-black” is occurring in the video sequence, every luminance value in the current frame may have changed relative to the previous frame, preventing the video encoder from finding a good match for the block. Fading also may occur in a sequence due to natural illumination changes. Blending and morphing, which are other transitioning effects, may also reduce the effectiveness of straightforward motion estimation/compensation.

D. Standards for Video Compression and Decompression

Aside from WMV8, several international standards relate to video compression and decompression. These standards include the Motion Picture Experts Group [“MPEG”] 1, 2, and 4 standards and the H.261, H.262, and H.263 standards from the International Telecommunication Union [“ITU”]. Like WMV8, these standards use a combination of intraframe and interframe compression, although the standards typically differ from WMV8 in the details of the compression techniques used. For example, Annex P of the H.263 standard describes a Reference Picture Resampling mode for use in prediction that can be used to adaptively alter the resolution of pictures during encoding.

Given the critical importance of video compression and decompression to digital video, it is not surprising that video compression and decompression are richly developed fields. Whatever the benefits of previous video compression and decompression techniques, however, they do not have the advantages of the following techniques and tools.

SUMMARY

One of the goals of video compression is to improve rate-distortion performance—in other words, to achieve the same quality using fewer bits, or to use the same amount of bits but achieve higher quality. One way to achieve this goal is to identify portions of video sequences that require relatively large amounts of bits to encode, and then find ways to better compress such portions. Portions of video sequences with effects such as fading, morphing, and blending are infrequent, but require relatively large amounts of bits to encode when they do occur because conventional motion-based video compression methods are generally not effective on such portions.

Accordingly, in summary, the detailed description is directed to various techniques and tools for improving rate-distortion performance for video sequences that include fade-ins, fade-outs, cross-fades, or other fading, blending, or morphing effects. For example, a video encoder performs fading compensation for a current frame by adjusting a reference frame based on fading parameters. This makes motion compensation using the reference frame more efficient. A video decoder performs the fading compensation by adjusting the reference frame. In particular, the detailed description addresses efficient ways to parameterize the reference frame adjustment.

In a first set of techniques and tools, a video encoder or decoder obtains fading parameters comprising a scaling parameter and a shifting parameter. The video encoder or decoder performs fading compensation by remapping pixel values (such as chrominance or luminance values) in a reference video image (such as a frame, field, or object plane) in terms of the fading parameters, which may be coded, for example, using fixed- or variable-length codes. In some cases, the scaling parameter is a contrast parameter and the shifting parameter is a brightness parameter. The fading parameters specify, for example, a pixel-wise linear transform comprising scaling pixel values (such as chrominance or luminance values) using the scaling parameter. In some cases, the pixel-wise linear transform further comprises shifting scaled pixel values using the shifting parameter. The video encoder or decoder performs motion compensation for at least part of a current video image relative to the remapped reference video image.

In a second set of techniques and tools, a video encoder or decoder obtains global luminance change compensation parameters consisting of a scaling parameter and a shifting parameter. The video encoder or decoder performs global luminance change compensation by remapping pixel values for all of a reference video frame in terms of the global luminance change compensation parameters. The video encoder or decoder performs motion compensation for at least part of a current video frame relative to the remapped reference video frame.

The various techniques and tools can be used in combination or independently. Different embodiments implement one or more of the described techniques and tools.

Additional features and advantages will be made apparent from the following detailed description of different embodiments that proceeds with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing block-based intraframe compression according to the prior art.

FIG. 2 is a diagram showing motion estimation in a video encoder according to the prior art.

FIG. 3 is a diagram showing block-based interframe compression according to the prior art.

FIG. 4 is a diagram showing block-based interframe decompression according to the prior art.

FIG. 5 is a block diagram of a suitable computing environment in which several described embodiments may be implemented.

FIG. 6 is a block diagram of a generalized video encoder system used in several described embodiments.

FIG. 7 is a block diagram of a generalized video decoder system used in several described embodiments.

FIG. 8 is a flowchart showing a technique for encoding video using fading estimation and compensation.

FIG. 9 is a flowchart showing a technique for decoding video encoded using fading compensation.

FIG. 10 is a block diagram of a video encoder system capable of performing fading estimation and compensation.

FIG. 11 is a block diagram of a video decoder system capable of performing fading compensation.

FIG. 12 is a flowchart showing a technique for fading detection and parameter extraction.

FIG. 13 is a chart showing a signaling scheme for indicating whether to use fading compensation for a frame.

DETAILED DESCRIPTION

Described embodiments relate to techniques and tools for fading estimation and/or compensation. Without fading compensation/estimation, video sequences with effects such as fading, morphing, and blending require relatively large amounts of bits to encode because conventional motion-based video compression methods are generally not effective on such frames. Described embodiments improve rate-distortion performance by performing fading estimation/compensation in such sequences. Various embodiments relate to techniques and tools for estimating, applying, coding and/or decoding global luminance change parameters.

Fading compensation in some embodiments involves performing a global luminance change to one or more reference frames to compensate for fading. A global luminance change is a luminance change across a defined region, which may be a frame, a part of a frame (e.g., an individual block or macroblock in a frame, or a group of macroblocks in a frame), or another specific portion of an image being coded or decoded. A current frame is then predicted by motion estimation/compensation from the adjusted one or more reference frames. Alternatively, fading compensation involves a global change to a reference frame to compensate for effects such as blending or morphing. Generally, fading compensation includes any compensation for fading (i.e., fade-to-black or fade-from-black), blending, morphing, or other natural or synthetic lighting effects that affect pixel value intensities. Without loss of generality, however, the terms global luminance change and fading are used interchangeably herein, unless the context clearly shows otherwise.

As an alternative to performing fading compensation on frames, some embodiments perform fading compensation on fields, object layers or other images.

In some embodiments, fading compensation occurs by adjusting pixel values in the luminance and chrominance channels of a reference frame in YUV color space. The adjustment includes scaling and shifting luminance pixel values and scaling and shifting chrominance pixel values. Alternatively, the color space is different (e.g., YIQ or RGB) and/or the compensation uses other adjustment techniques.

An encoder/decoder performs fading estimation/compensation on a frame-by-frame basis. Alternatively, an encoder/decoder performs fading estimation/compensation on some other basis or on a portion of a frame such as one or more blocks or macroblocks.

The various techniques and tools can be used in combination or independently. Different embodiments implement one or more of the described techniques and tools. Although the operations for these techniques are typically described in a particular, sequential order for the sake of presentation, it should be understood that this manner of description encompasses minor rearrangements in the order of operations, unless a particular ordering is required. For example, operations described sequentially may in some cases be rearranged or performed concurrently. Moreover, for the sake of simplicity, flowcharts typically do not show the various ways in which particular techniques can be used in conjunction with other techniques.

In some embodiments, the video encoder and decoder use various flags and signals in a bitstream. While specific flags and signals are described, it should be understood that this manner of description encompasses different conventions (e.g., 0's rather than 1's) for the flags and signals.

I. Computing Environment

FIG. 5 illustrates a generalized example of a suitable computing environment (500) in which several of the described embodiments may be implemented. The computing environment (500) is not intended to suggest any limitation as to scope of use or functionality, as the techniques and tools may be implemented in diverse general-purpose or special-purpose computing environments.

With reference to FIG. 5, the computing environment (500) includes at least one processing unit (510) and memory (520). In FIG. 5, this most basic configuration (530) is included within a dashed line. The processing unit (510) executes computer-executable instructions and may be a real or a virtual processor. In a multi-processing system, multiple processing units execute computer-executable instructions to increase processing power. The memory (520) may be volatile memory (e.g., registers, cache, RAM), non-volatile memory (e.g., ROM, EEPROM, flash memory, etc.), or some combination of the two. The memory (520) stores software (580) implementing an encoder or decoder, such as a video encoder or decoder.

A computing environment may have additional features. For example, the computing environment (500) includes storage (540), one or more input devices (550), one or more output devices (560), and one or more communication connections (570). An interconnection mechanism (not shown) such as a bus, controller, or network interconnects the components of the computing environment (500). Typically, operating system software (not shown) provides an operating environment for other software executing in the computing environment (500), and coordinates activities of the components of the computing environment (500).

The storage (540) may be removable or non-removable, and includes magnetic disks, magnetic tapes or cassettes, CD-ROMs, DVDs, or any other medium which can be used to store information and which can be accessed within the computing environment (500). The storage (540) stores instructions for the software (580) implementing the encoder or decoder.

The input device(s) (550) may be a touch input device such as a keyboard, mouse, pen, or trackball, a voice input device, a scanning device, or another device that provides input to the computing environment (500). For audio or video encoding, the input device(s) (550) may be a sound card, video card, TV tuner card, or similar device that accepts audio or video input in analog or digital form, or a CD-ROM or CD-RW that reads audio or video samples into the computing environment (500). The output device(s) (560) may be a display, printer, speaker, CD-writer, or another device that provides output from the computing environment (500).

The communication connection(s) (570) enable communication over a communication medium to another computing entity. The communication medium conveys information such as computer-executable instructions, audio or video input or output, or other data in a modulated data signal. A modulated data signal is a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media include wired or wireless techniques implemented with an electrical, optical, RF, infrared, acoustic, or other carrier.

The techniques and tools can be described in the general context of computer-readable media. Computer-readable media are any available media that can be accessed within a computing environment. By way of example, and not limitation, with the computing environment (500), computer-readable media include memory (520), storage (540), communication media, and combinations of any of the above.

The techniques and tools can be described in the general context of computer-executable instructions, such as those included in program modules, being executed in a computing environment on a target real or virtual processor. Generally, program modules include routines, programs, libraries, objects, classes, components, data structures, etc. that perform particular tasks or implement particular abstract data types. The functionality of the program modules may be combined or split between program modules as desired in various embodiments. Computer-executable instructions for program modules may be executed within a local or distributed computing environment.

For the sake of presentation, the detailed description uses terms like “estimate,” “signal,” “encode,” and “decode” to describe computer operations in a computing environment. These terms are high-level abstractions for operations performed by a computer, and should not be confused with acts performed by a human being. The actual computer operations corresponding to these terms vary depending on implementation.

II. Generalized Video Encoder and Decoder

FIG. 6 is a block diagram of a generalized video encoder (600) and FIG. 7 is a block diagram of a generalized video decoder (700).

The relationships shown between modules within the encoder and decoder indicate the main flow of information in the encoder and decoder; other relationships are not shown for the sake of simplicity. In particular, FIGS. 6 and 7 usually do not show side information indicating the encoder settings, modes, tables, etc. used for a video sequence, frame, macroblock, block, etc. Such side information is sent in the output bitstream, typically after entropy encoding of the side information. The format of the output bitstream can be a Windows Media Video format or another format.

The encoder (600) and decoder (700) are block-based and use a 4:2:0 macroblock format with each macroblock including 4 luminance 8×8 luminance blocks (at times treated as one 16×16 macroblock) and two 8×8 chrominance blocks. Alternatively, the encoder (600) and decoder (700) are object-based, use a different macroblock or block format, or perform operations on sets of pixels of different size or configuration than 8×8 blocks and 16×16 macroblocks.

Depending on implementation and the type of compression desired, modules of the encoder or decoder can be added, omitted, split into multiple modules, combined with other modules, and/or replaced with like modules. In alternative embodiments, encoder or decoders with different modules and/or other configurations of modules perform one or more of the described techniques.

A. Video Encoder

FIG. 6 is a block diagram of a general video encoder system (600). The encoder system (600) receives a sequence of video frames including a current frame (605), and produces compressed video information (695) as output. Particular embodiments of video encoders typically use a variation or supplemented version of the generalized encoder (600).

The encoder system (600) compresses predicted frames and key frames. For the sake of presentation, FIG. 6 shows a path for key frames through the encoder system (600) and a path for forward-predicted frames. Many of the components of the encoder system (600) are used for compressing both key frames and predicted frames. The exact operations performed by those components can vary depending on the type of information being compressed.

A predicted frame [also called p-frame, b-frame for bi-directional prediction, or inter-coded frame] is represented in terms of prediction (or difference) from one or more other frames. A prediction residual is the difference between what was predicted and the original frame. In contrast, a key frame [also called i-frame, intra-coded frame] is compressed without reference to other frames.

If the current frame (605) is a forward-predicted frame, a motion estimator (610) estimates motion of macroblocks or other sets of pixels of the current frame (605) with respect to a reference frame, which is the reconstructed previous frame (625) buffered in the frame store (620). In alternative embodiments, the reference frame is a later frame or the current frame is bi-directionally predicted. The motion estimator (610) outputs as side information motion information (615) such as motion vectors. A motion compensator (630) applies the motion information (615) to the reconstructed previous frame (625) to form a motion-compensated current frame (635). The prediction is rarely perfect, however, and the difference between the motion-compensated current frame (635) and the original current frame (605) is the prediction residual (645). Alternatively, a motion estimator and motion compensator apply another type of motion estimation/compensation.

A frequency transformer (660) converts the spatial domain video information into frequency domain (i.e., spectral) data. For block-based video frames, the frequency transformer (660) applies a discrete cosine transform [“DCT”] or variant of DCT to blocks of the motion prediction residual data, producing blocks of DCT coefficients. Alternatively, the frequency transformer (660) applies another conventional frequency transform such as a Fourier transform or uses wavelet or subband analysis. In some embodiments, the frequency transformer (660) applies a frequency transform to blocks of spatial prediction residuals for key frames. The frequency transformer (660) can apply an 8×8, 8×4, 4×8, or other size frequency transforms.

A quantizer (670) then quantizes the blocks of spectral data coefficients. The quantizer applies uniform, scalar quantization to the spectral data with a step-size that varies on a frame-by-frame basis or other basis. Alternatively, the quantizer applies another type of quantization to the spectral data coefficients, for example, a non-uniform, vector, or non-adaptive quantization, or directly quantizes spatial domain data in an encoder system that does not use frequency transformations. In addition to adaptive quantization, the encoder (600) can use frame dropping, adaptive filtering, or other techniques for rate control.

When a reconstructed current frame is needed for subsequent motion estimation/compensation, an inverse quantizer (676) performs inverse quantization on the quantized spectral data coefficients. An inverse frequency transformer (666) then performs the inverse of the operations of the frequency transformer (660), producing a reconstructed prediction residual (for a predicted frame) or a reconstructed key frame. If the current frame (605) was a key frame, the reconstructed key frame is taken as the reconstructed current frame (not shown). If the current frame (605) was a predicted frame, the reconstructed prediction residual is added to the motion-compensated current frame (635) to form the reconstructed current frame. The frame store (620) buffers the reconstructed current frame for use in predicting the next frame. In some embodiments, the encoder applies a deblocking filter to the reconstructed frame to adaptively smooth discontinuities in the blocks of the frame.

The entropy coder (680) compresses the output of the quantizer (670) as well as certain side information (e.g., motion information (615), quantization step size). Typical entropy coding techniques include arithmetic coding, differential coding, Huffman coding, run length coding, LZ coding, dictionary coding, and combinations of the above. The entropy coder (680) typically uses different coding techniques for different kinds of information (e.g., DC coefficients, AC coefficients, different kinds of side information), and can choose from among multiple code tables within a particular coding technique.

The entropy coder (680) puts compressed video information (695) in the buffer (690). A buffer level indicator is fed back to bitrate adaptive modules. The compressed video information (695) is depleted from the buffer (690) at a constant or relatively constant bitrate and stored for subsequent streaming at that bitrate. Alternatively, the encoder system (600) streams compressed video information immediately following compression.

Before or after the buffer (690), the compressed video information (695) can be channel coded for transmission over the network. The channel coding can apply error detection and correction data to the compressed video information (695).

B. Video Decoder

FIG. 7 is a block diagram of a general video decoder system (700). The decoder system (700) receives information (795) for a compressed sequence of video frames and produces output including a reconstructed frame (705). Particular embodiments of video decoders typically use a variation or supplemented version of the generalized decoder (700).

The decoder system (700) decompresses predicted frames and key frames. For the sake of presentation, FIG. 7 shows a path for key frames through the decoder system (700) and a path for forward-predicted frames. Many of the components of the decoder system (700) are used for decompressing both key frames and predicted frames. The exact operations performed by those components can vary depending on the type of information being compressed.

A buffer (790) receives the information (795) for the compressed video sequence and makes the received information available to the entropy decoder (780). The buffer (790) typically receives the information at a rate that is fairly constant over time, and includes a jitter buffer to smooth short-term variations in bandwidth or transmission. The buffer (790) can include a playback buffer and other buffers as well. Alternatively, the buffer (790) receives information at a varying rate. Before or after the buffer (790), the compressed video information can be channel decoded and processed for error detection and correction.

The entropy decoder (780) entropy decodes entropy-coded quantized data as well as entropy-coded side information (e.g., motion information (715), quantization step size), typically applying the inverse of the entropy encoding performed in the encoder. Entropy decoding techniques include arithmetic decoding, differential decoding, Huffman decoding, run length decoding, LZ decoding, dictionary decoding, and combinations of the above. The entropy decoder (780) frequently uses different decoding techniques for different kinds of information (e.g., DC coefficients, AC coefficients, different kinds of side information), and can choose from among multiple code tables within a particular decoding technique.

If the frame (705) to be reconstructed is a forward-predicted frame, a motion compensator (730) applies motion information (715) to a reference frame (725) to form a prediction (735) of the frame (705) being reconstructed. For example, the motion compensator (730) uses a macroblock motion vector to find a macroblock in the reference frame (725). A frame buffer (720) stores previous reconstructed frames for use as reference frames. Alternatively, a motion compensator applies another type of motion compensation. The prediction by the motion compensator is rarely perfect, so the decoder (700) also reconstructs prediction residuals.

When the decoder needs a reconstructed frame for subsequent motion compensation, the frame store (720) buffers the reconstructed frame for use in predicting the next frame. In some embodiments, the encoder applies a deblocking filter to the reconstructed frame to adaptively smooth discontinuities in the blocks of the frame.

An inverse quantizer (770) inverse quantizes entropy-decoded data. In general, the inverse quantizer applies uniform, scalar inverse quantization to the entropy-decoded data with a step-size that varies on a frame-by-frame basis or other basis. Alternatively, the inverse quantizer applies another type of inverse quantization to the data, for example, a non-uniform, vector, or non-adaptive quantization, or directly inverse quantizes spatial domain data in a decoder system that does not use inverse frequency transformations.

An inverse frequency transformer (760) converts the quantized, frequency domain data into spatial domain video information. For block-based video frames, the inverse frequency transformer (760) applies an inverse DCT [“IDCT”] or variant of IDCT to blocks of the DCT coefficients, producing motion prediction residual data. Alternatively, the frequency transformer (760) applies another conventional inverse frequency transform such as a Fourier transform or uses wavelet or subband synthesis. In some embodiments, the inverse frequency transformer (760) applies an inverse frequency transform to blocks of spatial prediction residuals for key frames. The inverse frequency transformer (760) can apply an 8×8, 8×4, 4×8, or other size inverse frequency transforms.

III. Global Luminance Change Estimation/Compensation

Some described embodiments involve techniques and tools for estimating, applying, coding and/or decoding global luminance changes described by a small number of parameters. The tools and techniques allow the same subjective and objective quality of reconstructed video to be achieved at a lower bit rate. Each individual tool or technique implements one or more of 1) a global luminance change compensation scheme; 2) parameterization of global luminance parameters; 3) computationally efficient estimation/extraction of global luminance parameters; 4) low-cost signaling of frames with global luminance change; and 5) low-cost signaling of global luminance parameters. The luminance change may be global to a frame, a field, a part of a frame/field such as an individual block, individual macroblock, or group of macroblocks, or another specific portion of an image. While much of the following description addresses parameterization and compensation for luminance change in an entire frame, the same framework for luminance change compensation can be used to (a) determine presence of fading in a portion (e.g., an individual block or macroblock) of an image, (b) compute fading parameters within this portion, and (c) parameterize and transmit these fading parameters for the given portion of the image. These techniques can be repeated for multiple portions of the image.

A global luminance change (also known as “fading”) may be a change in the brightness and/or contrast of the scene. Typically, the change is linear, but fading can also be defined as including any smooth, nonlinear mapping within the same framework. Fading, morphing and blending are widely used in creation of video content for smoothing the evolution of video frames across scene transitions and for providing special effects. Further, certain sequences exhibit fading naturally due to changes in illumination. Video frames with effects such as fading, morphing, and blending require relatively large amounts of bits to encode with conventional motion-based video compression methods, which are generally not effective on such frames.

A. Global Luminance Change Compensation Scheme

FIG. 8 shows a technique (800) for encoding video using global luminance change compensation. An encoder such as the encoder (600) shown in FIG. 6 may perform the technique (800).

First, the encoder checks for fading (810) in a frame to be encoded, such as the current frame being encoded in a video sequence. If the encoder detects fading (810) for the frame, the encoder obtains fading parameters (820). For example, the encoder detects fading and obtains fading parameters as described below in Section C. Alternatively, the encoder uses a different technique to detect fading and/or obtain parameters. The encoder signals whether fading compensation is on or off, and, if on, signals the fading parameters as well.

If fading was detected for the frame, the encoder then performs fading compensation on one or more reference frames for the frame (830), for example, as described below in Section B. When the encoder performs fading compensation on multiple reference frames, the multiple reference frames may be before or after (in playback order) the frame being encoded. The encoder signals which portions of the frame being encoded are compensated from which of the multiple reference frames. For this signaling, the encoder may use signaling already used for reference frame selection in such systems. In some embodiments, the encoder may use both an original reference frame and a remapped reference frame in motion estimation/compensation for the frame to be encoded. The encoder may do this, for example, to encode a current frame that has both faded content and unfaded overlays.

After fading compensation, the encoder encodes the frame (840) using motion estimation/compensation from the adjusted reference frame(s). If the encoder does not detect fading, the encoder encodes the frame (840) without obtaining fading parameters or performing fading compensation. When encoding is done (850), the process ends.

FIG. 9 shows a technique (900) for decoding video encoded using fading compensation. A decoder such as the decoder (700) shown in FIG. 7 may perform the technique (900).

First, the decoder checks (910) whether fading is on or off for the frame to be decoded. One way to perform this checking is to check signaling information sent by an encoder. If the decoder determines that fading is on (910) for the frame, the decoder performs fading compensation (920). For example, the decoder gets fading parameters sent by the encoder and performs fading compensation (as done in the encoder) on one or more reference frames for the frame to be decoded.

The decoder then decodes the frame (930), for example, using motion compensation from the adjusted reference frame(s). If fading is off, the decoder decodes the frame (930) without performing fading compensation. When decoding is done (940), the process ends.

FIG. 10 shows an exemplary encoder framework (1000) for performing global luminance change compensation. In this framework (1000), the encoder conditionally remaps a reference frame using parameters obtained by fading estimation. The encoder performs remapping, or fading compensation, when the encoder detects fading with a good degree of certainty and consistency across the frame. Otherwise, fading compensation is an identity operation (i.e., output=input).

Referring to FIG. 10, the encoder compares a current frame (1010) with a reference frame (1020) using a fading detection module (1030) to determine whether fading is occurring. In some embodiments, the reference frame is the frame previous to the current frame in a video sequence. Alternatively, the reference frame is earlier than the previous frame or after the current frame. When multiple reference frames are used, the encoder may check for fading in each reference frame. The encoder produces a “fading on” or “fading off” signal (1040) based on the results of the fading detection.

If fading is on, the fading estimation module (1050) estimates fading parameters (1060) based on calculations performed on the current frame (1010) and the reference frame (1020). (Fading estimation details in some embodiments are discussed below in Section C.)

The fading compensation module (1070) uses the fading parameters (1060) to remap the reference frame (1020) (alternatively, multiple reference frames). The encoder can then use other encoder modules (1080) (e.g., motion estimation and compensation, frequency transformer, and quantization modules) to compress the frame. The encoder outputs motion vectors, residuals and other information (1090) that define the encoded current frame (1010). Aside from motion estimation/compensation with translational motion vectors, the framework for global luminance change compensation is applicable across a wide variety of motion compensation-based video codecs.

FIG. 11 shows an exemplary decoder framework (1100) for performing global luminance change compensation. The decoder produces a decoded current frame (1110). To decode an encoded fading-compensated frame, the decoder performs fading compensation on a previously decoded reference frame (1120) (alternatively, multiple reference frames) using a fading compensation module (1130).

The decoder performs fading compensation on the reference frame (1120) if the fading on/off signal (1140) indicates that fading is on for the encoded current frame (1110). The decoder performs fading compensation (as done in the encoder) by using the fading parameters (1150) obtained during fading estimation. Otherwise (if fading is off for the current frame), fading compensation is an identity operation (i.e., output=input).

The decoder can then use other decoder modules (1160) (e.g., motion compensation, inverse frequency transformer, and inverse quantization modules) to decompress the encoded frame using motion vectors, residuals and other information (1170) provided by the encoder.

B. Parameterization and Compensation

In video editing, synthetic fading is sometimes realized by applying a simple, pixel-wise linear transform to the luminance and chrominance channels. Likewise, cross-fading is sometimes realized as linear sums of two video sequences, with the composition changing over time. Accordingly, in some embodiments, an encoder such as one shown in the framework (1000) of FIG. 10 parameterizes fading (whether natural or synthetic) as a pixel-wise linear transform and parameterizes cross-fading as a linear sum, and a decoder such as one shown in the framework (1100) of FIG. 11 performs corresponding transforms.

Let I(n) be an image at frame n and I(n−1) be an image at the previous frame. Where motion is small, simple fading is modeled by the first order relationship in Equation 1. (The relation in Equation 1 is approximate because of possible motion in the video sequence.) I(n)≈C I(n−1)+B  (1), where the fading parameters B and C correspond to brightness and contrast, respectively. When nonlinear fading occurs, the first order component typically accounts for the bulk of the change.

Cross-fades from an image sequence U(n) to an image sequence V(n) can be modeled by the relationship in Equation 2. The relation in Equation 2 is approximate because of possible motion in the sequences.

$\begin{matrix} \begin{matrix} {{I(n)} \approx {{\alpha\;{nV}} + {\left( {1 - {\alpha\; n}} \right)U}}} \\ {\approx {{I\left( {n - 1} \right)} + {\alpha\left( {V - U} \right)}}} \\ {\approx \left\{ \begin{matrix} {\left( {1 - \alpha} \right){I\left( {n - 1} \right)}} & {n \approx 0} \\ {\left( {1 + \alpha} \right){I\left( {n - 1} \right)}} & {n \approx {1/\alpha}} \end{matrix} \right.} \end{matrix} & (2) \end{matrix}$ n≈0 represents the beginning of the cross-fade, and n≈1/α represents the end of the cross-fade. For cross-fades spanning several frames, α is small. At the start of the cross-fade, the nth frame is close to an attenuated (contrast<1) version of the n−1th frame. Towards the end, the nth frame is an amplified (contrast>1) version of the n−1th frame. In other words, at the beginning of the cross-fade, the nth frame can be modeled as the n−1th frame scaled by the contrast value 1−α, while at the end of the cross-fade, the nth frame can be modeled as the n−1th frame scaled by the contrast value 1+α. Equation 2 shows that at the beginning and end of the cross-fade, the encoder can obtain the nth frame by remapping a reference frame (e.g., the n−1th frame) using a linear rule (such as those shown in Equations 3 and 4 below).

The encoder carries out compensation of global luminance change by remapping a reference frame. The encoder remaps the reference frame on a pixel-by-pixel basis, or on some other basis. The original, un-remapped reference frame is essentially discarded (although in a multiple reference frame scenario, the un-remapped reference may be used as well).

The following linear rule, based on Equation 1, remaps the luminance values of the reference frame R to the remapped reference frame {circumflex over (R)} in terms of the two parameters B and C (corresponding to brightness and contrast of the scene): {circumflex over (R)}≈CR+B  (3) The luminance values of the reference frame are scaled (or, “weighted”) by the contrast value and shifted (i.e., by adding an offset) by the brightness value. For chrominance, the remapping follows the rule: {circumflex over (R)}≈C(R−μ)+μ  (4) where μ is the mean of the chrominance values. In one embodiment, 128 is assumed to be the mean for unsigned 8-bit representation of chrominance values. This rule for chrominance remapping does not use a brightness component.

In some embodiments, the two-parameter linear remapping used in Equations 3 and 4 is extended to higher order terms. For example, Equation 5 is a second-order equation that remaps the luminance values of R to {circumflex over (R)}: {circumflex over (R)}≈C₁R²+C₂R+B  (5) Other embodiments use other remapping rules. In one category of such remapping rules, for non-linear fading, linear mappings are replaced with non-linear mappings.

C. Estimation of Fading Parameters

Fading estimation is the process of computing fading parameters during the encoding process. An encoder such as one shown in the framework (1000) of FIG. 10 may compute brightness (B) and contrast (C) parameters during the encoding process.

In some embodiments, in order to estimate parameters accurately and in a speedy manner, the encoder uses the technique (1200) illustrated in FIG. 12. In the illustrated technique, only the luminance channel is analyzed. Alternatively, the encoder includes chrominance in the analysis when more computational resources are available. For example, the encoder solves for C in Equations 3 and 4 (not just Equation 3) to make C more robust.

In the embodiment illustrated in FIG. 12, motion in the scene is ignored during the fading estimation process. This is based on the observations that (a) fades and cross fades typically happen at still or low-motion scenes and (b) the utility of global luminance change compensation in high motion scenes is very low. Alternatively, the encoder jointly solves for fading parameters and motion information. Motion information is then used to refine the accuracy of fading parameters at the later stages of the technique (1200) or at some other time. One way to use motion information is to omit from the fading estimation computation those portions of the reference frame in which movement is detected.

In various parts of the technique (1200), the absolute error sums of Σabs(I(n)−R) or Σabs(I(n)−{circumflex over (R)}) serve as metrics for determining the existence and parameters of fading. Alternatively, the encoder uses other or additional metrics such as sum of squared errors [“SSE”] or mean squared error [“MSE”] over the same error term, or the encoder uses a different error term.

At various points during the technique (1200), the encoder may end the technique (1200) upon satisfaction of an exit condition. FIG. 12 shows several exit conditions. For another exit condition (not shown in FIG. 12), the encoder checks whether the contrast parameter is close to 1.0 (in one implementation, 0.99<C<1.02) at the start or at an intermediate stage of the technique (1200) and, if so, ends the technique (1200).

The encoder begins the process (1200) by downsampling the current frame and the reference frame (1210). In one implementation, the encoder downsamples by a factor of 4 horizontally and vertically. Alternatively, the encoder downsamples by another factor, or does not downsample at all.

The encoder then computes the absolute error sum Σabs(I_(d)(n)−R_(d)) over the lower-resolution versions of the current and reference frames (indicated by the subscript d) (1220). The absolute error sum measures differences in pixel values between the downsampled current frame and the downsampled reference frame. If the absolute error sum is smaller than a certain threshold (1230) (e.g., a pre-determined difference measure between luminance values for pixels in the downsampled current and reference frames), the encoder concludes that no fading has occurred and does not perform fading compensation (1235).

Otherwise, the encoder estimates brightness (B) and contrast (C) parameters (1240). First cut estimates for B and C are obtained by modeling I_(d)(n) in terms of R_(d). In one embodiment, the brightness and contrast parameters are obtained through linear regression over the entire downsampled frame. In other embodiments, the encoder uses other forms of statistical analysis such as total least squares, least median of squares, etc. for more robust analysis. For example, the encoder minimizes the MSE or SSE of the error term I_(d)(n)−R_(d). In some circumstances, MSE and SSE are not robust, so the encoder also tests the absolute error sum for the error term. The encoder discards high error values for particular points (which may be due to motion rather than fading).

The encoder then calculates B_(f) and C_(f) by quantizing and dequantizing B and C (1250). The first cut parameters are quantized and dequantized, giving B_(f) and C_(f), to ensure that they lie within the permissible range and to test for compliance. In some embodiments, for typical 8-bit depth imagery, B and C are quantized to 6 bits each. B takes on integer values from −32 to 31 represented as a signed 5-bit integer. The quantized value of C, represented as C_(q), varies from 0.515625 to 1.484375, in uniform steps of 0.015625 ( 1/64), corresponding to quantized values 1 through 63. Quantization is performed by rounding B and C to the nearest valid dequantized value and picking the appropriate bin index.

The encoder then calculates the original bounded absolute error sum (S_(OrgBnd)) and remapped bounded absolute error sum (S_(RmpBnd)) (1270). In some embodiments, the encoder calculates the sums using a goodness-of-fit analysis. For a random or pseudorandom set of pixels at the original resolution, the encoder computes the remapped bounded absolute error sum Σbabs(I(n)−C_(f)R−B_(f)), where babs(x)=min(abs(x), M) for some bound M. In one implementation, M is a multiple of the quantization parameter of the frame being encoded. The bound M is higher when the quantization parameter is coarse, and lower when the quantization parameter is fine. The encoder also accumulates the original bounded absolute error sum Σbabs(I(n)−R). If computational resources are available, the encoder may compute the bounded error sums over the entire frames.

Then, based on the relative values of the original and remapped bounded absolute error sum, the encoder determines whether or not to use fading compensation (1280). For example, in some embodiments, the encoder does not perform fading compensation unless the remapped bounded absolute error sum is less than or equal to some threshold percentage σ of the original bounded absolute error sum. In one implementation, σ=0.95. If the encoder is performing fading compensation, the encoder recomputes the fading parameters, this time based on a linear regression between I(n) and R, but at the full resolution (1290). To save computation time, the encoder can perform the repeated linear regression over the random or pseudorandom sampling of the frame. Again, the encoder can alternatively use other forms of statistical analysis (e.g., total least squares, least median of squares, etc.) for more robust analysis. When encoding is done (1295), the process ends.

In one embodiment, the encoder allows a special case in which C=−1 in Equations 3 and 4. The special case is signaled by C_(q)=0 in the bitstream. In this “invert” mode, the reference frame is inverted before shifting by B, and the range of B is 193 to 319 in uniform steps of two.

D. Signaling

An encoder such as one shown in the framework (1000) of FIG. 10 sends fading on/off signaling information as well as fading parameters. A decoder such as one shown in the framework (1100) of FIG. 11 receives the signaling information and fading parameters.

In some embodiments, at the sequence level, the encoder sends one bit to indicate whether global luminance change compensation is enabled for the sequence. The encoder also can signal global luminance change at the frame level and/or signal global luminance change for a portion of a frame, such as an individual block or macroblock in a frame.

Among the frames in a typical video sequence, the occurrence of global luminance change or fading is rare. It is possible to signal fading (or, equivalently, the absence of fading) by adding a bit. For example, the encoder can signal fading (or, equivalently, the absence of fading) at the frame level by using one bit. However, it is more economical to signal fading (or, equivalently, the absence of fading) jointly with other information. For example, the encoder performs frame-level signaling using an escape sequence in a variable length code (VLC) table for motion mode (i.e., the number and configuration of motion vectors, the sub-pixel interpolation scheme). In such embodiments, the encoder jointly signals the least frequent motion mode and the activation of fading compensation.

Let the event F represent the existence of fading and G represent the choice of the least frequent motion mode for the frame. Let the VLC <MVMODE> represent a motion mode when G is false. This VLC table is designed with an escape symbol <ESC> used to signal when F and/or G is true. Table 1 illustrates the joint coding of F and the motion mode, followed by the fading parameters B and C when fading compensation is activated.

TABLE 1 Joint coding of least frequent motion mode and fading signal G = false G = true F = false <MVMODE> <ESC> 0 F = true <ESC> 1 <MVMODE>[B][C] <ESC> 1 <ESC>[B][C] If global luminance change is activated for a certain frame, another VLC (either <MVMODE> or <ESC>) follows to indicate the motion mode. Then, the parameters are signaled using two 6-bit fixed-length code words for B and C, respectively. Alternatively, the parameters are signaled using VLCs. When applied to an individual portion of a frame, such as a block or a macroblock, the encoder can signal the fading parameters independently with that portion. For example, if fading applies to only a macroblock of the video frame, the fading information can be signaled by joint entropy codes with macroblock-level information such as the coded block pattern or transform type.

FIG. 13 is a tree diagram showing a coding arrangement (corresponding to Table 1, for one implementation) where the encoder jointly codes the least frequent motion mode and the existence of fading. (Details of coding for fading parameters are omitted.) The encoder uses VLCs to represent each of the other motion modes (e.g., the codes “0,” “10,” and “110” represent motion modes 0, 1 and 2, respectively). The encoder uses an escape code (e.g., “111”) followed by a “0” to represent the least frequent motion mode when fading is not activated. The encoder uses the escape code followed by a “1” to indicate that fading is activated.

It should be understood that the programs, processes, or methods described herein are not related or limited to any particular type of computing environment, unless indicated otherwise. Various types of general purpose or specialized computing environments may be used with or perform operations in according with the teachings described herein. Elements of embodiments shown in software may be implemented in hardware and vice versa.

In view of the many possible embodiments to which the principles of our invention may be applied, we claim as our invention all such embodiments as may come within the scope and the spirit of the following claims and equivalents thereto. 

We claim:
 1. A computer system comprising: one or more processing units; memory; and storage storing computer-executable instructions for causing the one or more processing units, when programmed thereby, to perform operations comprising: receiving a current frame of a video sequence, the current frame having multiple reference frames; encoding the current frame, thereby producing encoded data, wherein the encoding the current frame includes: determining to activate fading compensation for the current frame; encoding, in a video bitstream, a value of a frame-level on/off signal that indicates fading compensation is activated for the current frame; and encoding a portion of the current frame, wherein the encoding the portion of the current frame includes, for each of the multiple reference frames: determining fading compensation parameters for the portion of the current frame relative to the reference frame; encoding, in the video bitstream, the fading compensation parameters for the portion of the current frame relative to the reference frame; and adjusting multiple pixel values from the reference frame based at least in part on the fading compensation parameters for the portion of the current frame relative to the reference frame; and outputting, as part of the video bitstream, the encoded data.
 2. The computer system of claim 1, wherein the multiple portions are groups of macroblocks of the current frame.
 3. The computer system of claim 1, wherein the frame-level on/off signal is a one-bit flag value.
 4. The computer system of claim 1, wherein the encoding the portion of the current frame further includes encoding, in the video bitstream, information indicating which of the reference frames are adjusted for fading compensation for the portion of the current frame.
 5. The computer system of claim 1, wherein the encoding the portion of the current frame further includes encoding, in the video bitstream, information indicating which of the reference frames are used in motion compensation for respective macroblocks of the portion of the current frame.
 6. The computer system of claim 1, wherein the adjusting the multiple pixel values from the reference frame uses a linear mapping for a given pixel value, the linear mapping including multiplying the given pixel value by a scaling parameter and adding a shifting parameter to a result of the multiplying.
 7. The computer system of claim 1, wherein the encoding the fading compensation parameters for the portion of the current frame relative to the reference frame includes entropy coding the fading compensation parameters for the portion of the current frame relative to the reference frame.
 8. The computer system of claim 1, wherein the multiple pixel values in the reference frame include luminance values or chrominance values.
 9. In a computer system, a method comprising: receiving, as part of a video bitstream, encoded data for at least part of a video sequence; and decoding, using at least some of the encoded data, a current frame of the video sequence, the current frame having multiple reference frames, wherein the decoding the current frame includes: decoding a value of a frame-level on/off signal that indicates fading compensation is activated for the current frame; determining to activate fading compensation for the current frame; and decoding a portion of the current frame, wherein the decoding the portion of the current frame includes, for each of the reference frames: decoding the fading compensation parameters for the portion of the current frame relative to the reference frame; and adjusting multiple pixel values from the reference frame based at least in part on the fading compensation parameters for the portion of the current frame relative to the reference frame.
 10. The method of claim 9, wherein the multiple portions are groups of macroblocks of the current frame.
 11. The method of claim 9, wherein the frame-level on/off signal is a one-bit flag value.
 12. The method of claim 9, wherein the decoding the portion of the current frame further includes decoding, from the video bitstream, information indicating which of the reference frames are adjusted for fading compensation for the portion of the current frame.
 13. The method of claim 9, wherein the decoding the portion of the current frame further includes decoding, from the video bitstream, information indicating which of the reference frames are used in motion compensation for respective macroblocks of the portion of the current frame.
 14. The method of claim 9, wherein the adjusting the multiple pixel values from the reference frame uses a linear mapping for a given pixel value, the linear mapping including multiplying the given pixel value by a scaling parameter and adding a shifting parameter to a result of the multiplying.
 15. The method of claim 9, wherein the decoding the fading compensation parameters for the portion of the current frame relative to the reference frame includes entropy decoding the fading compensation parameters for the portion of the current frame relative to the reference frame.
 16. The method of claim 9, wherein the multiple pixel values in the reference frame include luminance values or chrominance values.
 17. One or more non-transitory computer-readable media having stored thereon computer-executable instructions for causing one or more processing units, when programmed thereby, to perform operations, the operations comprising: receiving, as part of a video bitstream, encoded data for at least part of a video sequence; and decoding, using at least some of the encoded data, a current frame of the video sequence, the current frame having multiple reference frames, wherein the decoding the current frame includes: decoding a value of a frame-level on/off signal that indicates fading compensation is activated for the current frame; determining to activate fading compensation for the current frame; and decoding a portion of the current frame, wherein the decoding the portion of the current frame includes, for each of the reference frames: decoding the fading compensation parameters for the portion of the current frame relative to the reference frame; and adjusting multiple pixel values from the reference frame based at least in part on the fading compensation parameters for the portion of the current frame relative to the reference frame.
 18. The one or more computer-readable media of claim 17, wherein the decoding the portion of the current frame further includes decoding, from the video bitstream, information indicating which of the reference frames are adjusted for fading compensation for the portion of the current frame.
 19. The one or more computer-readable media of claim 17, wherein the decoding the portion of the current frame further includes decoding, from the video bitstream, information indicating which of the reference frames are used in motion compensation for respective macroblocks of the portion of the current frame.
 20. The one or more computer-readable media of claim 17, wherein the adjusting the multiple pixel values from the reference frame uses a linear mapping for a given pixel value, the linear mapping including multiplying the given pixel value by a scaling parameter and adding a shifting parameter to a result of the multiplying. 